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Plasma etching
Plasma etching
Plasma etching
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Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral (atoms and radicals). During the process, the plasma generates volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.[1]

Mechanisms

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Plasma generation

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A plasma is a high energetic condition in which a lot of processes can occur. These processes happen because of electrons and atoms. To form the plasma electrons have to be accelerated to gain energy. Highly energetic electrons transfer the energy to atoms by collisions. Three different processes can occur because of this collisions:[2][3]

Different species are present in the plasma such as electrons, ions, radicals, and neutral particles. Those species are interacting with each other constantly. Two processes occur during plasma etching:[4]

  • generation of chemical species
  • interaction with the surrounding surfaces

Without a plasma, all those processes would occur at a higher temperature. There are different ways to change the plasma chemistry and get different kinds of plasma etching or plasma depositions. One way to form a plasma is by using RF excitation by a power source of 13.56 MHz, a frequency allocated for this application in the ISM bands.

The mode of operation of the plasma system will change if the operating pressure changes. Also, it is different for different structures of the reaction chamber. In the simple case, the electrode structure is symmetrical, and the sample is placed upon the grounded electrode.

Influences on the process

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The key to develop successful complex etching processes is to find the appropriate gas etch chemistry that will form volatile products with the material to be etched as shown in Table 1.[3] For some difficult materials (such as magnetic materials), the volatility can only be obtained when the wafer temperature is increased. The main factors that influence the plasma process:[2][3][5]

  • Electron source
  • Pressure
  • Gas species
  • Vacuum

Surface interaction

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The reaction of the products depend on the likelihood of dissimilar atoms, photons, or radicals reacting to form chemical compounds. The temperature of the surface also affects the reaction of products. Adsorption happens when a substance is able to gather and reach the surface in a condensed layer, ranging in thickness (usually a thin, oxidized layer.) Volatile products desorb in the plasma phase and help the plasma etching process as the material interacts with the sample's walls. If the products are not volatile, a thin film will form at the surface of the material. Different principles that affect a sample's ability for plasma etching:[3][6]

Plasma etching can change the surface contact angles, such as hydrophilic to hydrophobic, or vice versa. Argon plasma etching has reported to enhance contact angle from 52 deg to 68 deg,[7] and, Oxygen plasma etching to reduce contact angle from 52 deg to 19 deg for CFRP composites for bone plate applications. Plasma etching has been reported to reduce the surface roughness from hundreds of nanometers to as much lower as 3 nm for metals.[8]

Types

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Pressure influences the plasma etching process. For plasma etching to happen, the chamber has to be under low pressure, less than 100 Pa. In order to generate low-pressure plasma, the gas has to be ionized. The ionization happens by a glow charge. Those excitations happen by an external source, which can deliver up to 30 kW and frequencies from 50 Hz (dc) over 5–10 Hz (pulsed dc) to radio and microwave frequency (MHz-GHz).[2][9]

Microwave plasma etching

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Microwave etching happens with an excitation sources in the microwave frequency, so between MHz and GHz. One example of plasma etching is shown here.[10]

A microwave plasma etching apparatus. The microwave operates at 2.45 GHz. This frequency is generated by a magnetron and discharges through a rectangular and a round waveguide. The discharge area is in a quartz tube with an inner diameter of 66mm. Two coils and a permanent magnet are wrapped around the quartz tube to create a magnetic field which directs the plasma.

Hydrogen plasma etching

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One form to use gas as plasma etching is hydrogen plasma etching. Therefore, an experimental apparatus like this can be used:[5]

A quartz tube with an rf excitation of 30 MHz is shown. It is coupled with a coil around the tube with a power density of 2-10 W/cm³. The gas species is H2 gas in the chamber. The range of the gas pressure is 100-300 um.

Plasma etcher

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A plasma etcher, or etching tool, is a tool used in the production of semiconductor devices. A plasma etcher produces a plasma from a process gas, typically oxygen or a fluorine-bearing gas, using a high frequency electric field, typically 13.56 MHz. A silicon wafer is placed in the plasma etcher, and the air is evacuated from the process chamber using a system of vacuum pumps. Then a process gas is introduced at low pressure, and is excited into a plasma through dielectric breakdown.

Plasma confinement

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Industrial plasma etchers often feature plasma confinement to enable repeatable etch rates and precise spatial distributions in RFTooltip radio frequency plasmas.[11] One method of confining plasmas is by using the properties of the Debye sheath, a near-surface layer in plasmas similar to the double layer in other fluids. For example, if the Debye sheath length on a slotted quartz part is at least half the width of the slot, the sheath will close off the slot and confine the plasma, while still permitting uncharged particles to pass through the slot.

Applications

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Plasma etching is currently used to process semiconducting materials for their use in the fabrication of electronics. Small features can be etched into the surface of the semiconducting material in order to be more efficient or enhance certain properties when used in electronic devices.[3] For example, plasma etching can be used to create deep trenches on the surface of silicon for uses in microelectromechanical systems. This application suggests that plasma etching also has the potential to play a major role in the production of microelectronics.[3] Similarly, research is currently being done on how the process can be adjusted to the nanometer scale.[3]

Hydrogen plasma etching, in particular, has other interesting applications. When used in the process of etching semiconductors, hydrogen plasma etching has been shown to be effective in removing portions of native oxides found on the surface.[5] Hydrogen plasma etching also tends to leave a clean and chemically balanced surface, which is ideal for a number of applications.[5]

Oxygen plasma etching can be used for anisotropic deep-etching of diamond nanostructures by application of high bias in inductively coupled plasma/reactive ion etching (ICP/RIE) reactor.[12] On the other hand, the use of oxygen 0V bias plasmas can be used for isotropic surface termination of C-H terminated diamond surface.[13]

Integrated circuits

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Plasma can be used to grow a silicon dioxide film on a silicon wafer (using an oxygen plasma), or can be used to remove silicon dioxide by using a fluorine bearing gas. When used in conjunction with photolithography, silicon dioxide can be selectively applied or removed to trace paths for circuits.

For the formation of integrated circuits it is necessary to structure various layers. This can be done with a plasma etcher. Before etching, a photoresist is deposited on the surface, illuminated through a mask, and developed. The dry etch is then performed so that structured etching is achieved. After the process, the remaining photoresist has to be removed. This is also done in a special plasma etcher, called an asher.[14]

Dry etching allows a reproducible, uniform etching of all materials used in silicon and III-V semiconductor technology. By using inductively coupled plasma/reactive ion etching (ICP/RIE), even hardest materials like e.g. diamond can be nanostructured.[15][16]

Plasma etchers are also used for de-layering integrated circuits in failure analysis.

Printed circuit boards

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Plasma is used to etch printed circuit boards, including de-smear vias.[17]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Plasma etching

View on Grokipedia
from Grokipedia
Plasma etching is a dry etching technique widely employed in microfabrication to precisely remove material from a substrate surface by exposing it to a plasma containing reactive species, such as ions, radicals, and neutral atoms, which facilitate chemical reactions and physical bombardment. This process occurs in a low-pressure chamber where a gas mixture, typically including etchants like CF₄, Cl₂, or C₄F₈, along with inert gases such as Ar or He, is ionized using radiofrequency (RF) power to generate the plasma. Unlike wet etching, plasma etching enables anisotropic etching profiles, crucial for defining high-aspect-ratio features in modern devices. The development of plasma etching began in the late 1960s as an alternative to wet chemical methods for (IC) fabrication, initially focusing on photoresist stripping and isotropic etching of materials like and . By the early 1970s, capacitively coupled planar systems were introduced, leveraging bombardment to achieve greater and reduce undercutting, which marked a pivotal shift toward precise transfer in semiconductor manufacturing. Key advancements in the included dual-frequency RF systems and inductively coupled plasmas (ICP), allowing independent control of plasma density and energy for improved uniformity and selectivity. At its core, plasma etching involves multiple mechanisms: pure chemical etching driven by reactive radicals, physical sputtering from accelerated ions, and ion-enhanced chemical reactions that synergistically boost etch rates and directionality. Process parameters, such as pressure (typically 0.001–10 Torr), RF power, gas composition, and substrate temperature, are finely tuned to optimize outcomes like etch rate, selectivity to masks (e.g., photoresist or oxide), and surface roughness. Common variants include reactive ion etching (RIE), which emphasizes ion bombardment for anisotropy, and downstream etching for safer, radical-dominated processes. In semiconductor manufacturing, plasma etching is indispensable for fabricating nanoscale features in ICs, enabling the production of transistors at advanced nodes like 2 nm and high-density interconnects. Beyond electronics, it supports applications in microelectromechanical systems (MEMS), photonics, and advanced materials processing, where precision and minimal residue are paramount. Challenges persist, including achieving sub-nanometer uniformity, managing defects from plasma-induced damage, and addressing environmental concerns from fluorinated gases, driving ongoing innovations like atomic layer etching (ALE) and advanced conductor etch tools (as of 2025) for next-generation devices.

Fundamentals

Definition and Principles

Plasma etching is a dry etching technique employed in microfabrication to selectively remove material from a substrate surface by utilizing a plasma, which is an ionized gas consisting of reactive ions, radicals, electrons, and neutral . This process involves both chemical reactions, where reactive species interact with the surface to form volatile byproducts, and physical by energetic ions that enhance material removal. Unlike wet etching methods that rely on chemicals and typically result in isotropic etching, plasma etching operates in a vacuum environment and can achieve anisotropic profiles, making it essential for precise pattern transfer in semiconductor manufacturing. The basic principles of plasma etching center on the ionization of etchant gases, such as tetrafluoromethane (CF₄) or (SF₆), within a low-pressure chamber to generate the necessary reactive species. Electrons in the plasma collide with gas , leading to dissociation and the production of radicals and ions that drive the process; for instance, fluorine radicals from CF₄ react with to form volatile (SiF₄). The general equation for plasma dissociation illustrates this initial step:
e−+AB→A∙+B∙+e−e^- + AB \rightarrow A^\bullet + B^\bullet + e^-
where e−e^- represents an and AB is the etchant , yielding reactive radicals A∙A^\bullet and B∙B^\bullet. Radicals primarily facilitate chemical through surface reactions, while ions provide directional control via momentum transfer, and electrons maintain the plasma discharge. This technique plays a pivotal role in by enabling the fabrication of sub-micron features in integrated circuits, where traditional wet methods fall short in resolution and control. By combining chemical selectivity with physical enhancement, plasma etching supports the scaling of device dimensions, contributing to advancements in and microelectromechanical systems ().

Historical Development

Plasma etching emerged in the mid-1960s as a dry processing technique for semiconductor manufacturing, extending physical methods to enable more precise material removal compared to traditional . Early pioneering work at Bell Laboratories included the development of RF plasma systems for etching and other materials, with M.P. Lepselter filing a key patent in 1969 that described plasma-based etching processes for integrated circuits. This innovation addressed limitations in , such as undercutting and environmental concerns, and was initially applied to stripping and isotropic etching of , , and metals like aluminum. By the late and early , plasma etching transitioned from research to production, with capacitively coupled RF discharges facilitating isotropic etching in barrel reactors at pressures around 1 . The decade's major milestone was the invention of (RIE) in the mid-1970s, which combined chemical reactivity with physical ion bombardment to achieve anisotropic profiles essential for finer features. Seminal contributions included N. Hosokawa's 1974 demonstration using fluoro-chloro-hydrocarbon gases and over a dozen RIE patents filed worldwide by 1975, including work by A.R. Reinberg on selective etching chemistries. These advancements, building on earlier patents by S.M. Irving from 1968–1971, reduced lateral etching and improved uniformity in device fabrication. Commercialization surged in the , driven by the need for scalable tools in high-volume . , founded in 1980, introduced the AutoEtch 480 in 1981—the industry's first fully automated, single-wafer plasma etcher—enabling precise control and higher throughput for polysilicon and . This period also saw the adoption of planar and systems for better , alongside polymerizing gas mixtures to enhance selectivity. The 1990s marked a shift toward advanced anisotropic techniques to support very large-scale integration (VLSI), with plasma etching enabling feature sizes below 1 micron through optimized RIE and magnetically enhanced variants. Post-2000, integration with deep ultraviolet lithography further refined etching precision, sustaining Moore's Law by allowing transistor densities to double roughly every two years; without plasma etching's directional control, scaling would have stalled around 1980 at 1-micron dimensions.

Mechanisms

Plasma Generation

Plasma generation in etching systems primarily relies on electrical discharges to ionize gases, creating a partially ionized medium essential for the etching process. The most common method is radio-frequency (RF) , typically operating at 13.56 MHz, which is an industrial standard due to its efficiency in sustaining stable plasmas at low pressures. In this mode, RF power is applied between parallel electrodes, accelerating electrons to collide with gas molecules and initiate . Direct current (DC) represents an earlier approach, where a steady voltage across electrodes generates a plasma through cathode fall regions, though it is less favored in modern etching due to electrode erosion issues. excitation, often at 2.45 GHz, provides an electrodeless alternative, coupling power directly into the gas via electromagnetic waves to produce uniform, high-density plasmas suitable for large-area processing. Key plasma properties in these etching systems include electron temperatures ranging from 1 to 10 eV and ion densities of 10^9 to 10^12 cm^{-3}, which ensure a non-equilibrium state where electrons are energetic while ions and neutrals remain near . Plasma initiation requires overcoming the , governed by , where the minimum VbV_b depends on the product of gas pp and electrode gap dd, typically expressed as Vb=f(p⋅d)V_b = f(p \cdot d). This relationship determines the conditions for stable discharge, with optimal occurring at specific p⋅dp \cdot d values around 0.1 to 1 ·cm for common etching gases. Gas selection plays a critical role, with inert gases like argon used for initial plasma striking due to their low ionization energies, while reactive gases such as CF_4 or Cl_2 are introduced for etching specificity; operations occur at low pressures of 1 to 100 mTorr to maintain non-equilibrium conditions and minimize collisions that could thermalize the plasma. Plasma sustenance involves continuous power coupling, either capacitively through electric fields in RF systems or inductively via magnetic fields in advanced setups like inductively coupled plasmas (ICP), where the primary mechanism is electron-impact ionization to replenish lost charges. These methods ensure sustained ionization rates, with electron collisions providing the energy to maintain the required densities without excessive heating of the substrate.

Chemical and Physical Etching Processes

In plasma , chemical processes dominate material removal through reactions between reactive radicals generated in the plasma and the substrate surface, leading to the formation and desorption of volatile byproducts. These radicals, such as atomic (F•), adsorb onto the surface, undergo bond-breaking and reformation, and produce gases that evacuate without residue. A example is the etching of , where four fluorine atoms react with a silicon atom to form :
\ceSi+4F∙−>SiF4(g)\ce{Si + 4F^\bullet -> SiF4 (g)}
This proceeds via sequential fluorination of the surface, with SiF₄ desorbing as the primary product, though minor contributions from SiF₂ may occur under certain conditions. The reaction exhibits a low of approximately 0.1 eV for initial F adsorption, but desorption of fluorinated species requires higher energies around 0.65 eV, influencing overall kinetics. Reaction rates are flux-dependent, with the etching probability per incident F atom typically ranging from 0.001 to 0.06, decreasing at high fluxes (>10¹⁸ cm⁻² s⁻¹) due to surface passivation by SiF radicals. Physical etching mechanisms rely on ion bombardment from the plasma, where accelerated s transfer to surface atoms, ejecting them via without chemical alteration. This process is quantified by the yield YY, the average number of target atoms removed per incident , which according to Sigmund's depends on the EE, target , and . The yield is approximately
Y≈0.042Sn(E)UsY \approx 0.042 \frac{S_n(E)}{U_s}
where Sn(E)S_n(E) represents the nuclear (a measure of energy transfer efficiency through elastic collisions), and UsU_s is the surface (typically the heat of sublimation). Thus, YY scales with energy transfer efficiency and inversely with , with typical values for keV s on semiconductors ranging from 0.1 to 1 atom/, though yields drop sharply below ~20-50 eV threshold energies. Synergistic effects between chemical and physical processes dramatically enhance etch rates, often by orders of magnitude beyond additive contributions, primarily through ion-assisted chemical etching that promotes product desorption and enables directional control. Energetic ions (~10-500 eV) disrupt surface bonds or fluorinated layers, facilitating radical reactions that would otherwise be kinetically limited, as shown in beam experiments where combined XeF₂ neutral flux and Ar⁺ ions etched silicon 20-100 times faster than either alone. This synergy underpins anisotropy by confining enhanced etching to ion-impact directions, while the Langmuir adsorption model describes precursor sticking and site availability, with surface coverage θ\theta given by
θ=sΓsΓ+ν\theta = \frac{s \Gamma}{s \Gamma + \nu}
where ss is the sticking coefficient, Γ\Gamma the radical flux, and ν\nu the desorption rate. The resulting etch rate follows R=k[radical](1−θ)R = k [\text{radical}] (1 - \theta), reflecting available bare sites for reaction amid partial coverage.

Surface Interactions and Selectivity

In plasma etching, surface interactions primarily involve the bombardment of substrate surfaces by charged ions and neutral radicals generated in the plasma. Ions interact through direct implantation, where they penetrate the surface lattice, causing physical or enhancing chemical reactions by breaking bonds and facilitating volatile product formation. In contrast, neutral radicals adsorb onto the surface, leading to spontaneous chemical via formation of volatile compounds without requiring energetic bombardment. These distinct mechanisms allow for tailored etching behaviors, with ion implantation promoting in directional processes, while radical adsorption drives isotropic chemical removal. Ion reflection coefficients, which quantify the fraction of incident ions that bounce off the surface rather than implanting, play a critical role in determining etching uniformity, particularly in high-aspect-ratio features. These coefficients depend on ion energy, incidence angle, and surface material, typically ranging from 0 to 1, with lower values indicating higher implantation efficiency. Low reflection coefficients aid in minimizing sidewall scattering and preserving profile fidelity in chlorine-based plasmas on silicon. Etch selectivity, defined as S=RsubstrateRmaskS = \frac{R_{\text{substrate}}}{R_{\text{mask}}}, where RR denotes etch rate, is governed by differences in surface reactivity between the target substrate and masking materials. Mask materials like photoresists offer moderate selectivity due to their organic composition, which etches slower than inorganic substrates in plasmas, while (SiO₂) masks provide higher durability in -based chemistries owing to the formation of stable passivation layers. A representative example is the Si/ etch ratio of approximately 100:1 in plasmas at cryogenic temperatures around -30°C, achieved through high radical concentrations that preferentially volatilize silicon as SiF₄ while passivating SiO₂. This selectivity enables precise pattern transfer without excessive mask erosion. Uniformity challenges arise from plasma-induced charging, where differential electron and fluxes accumulate on insulating surfaces or features, distorting local and causing deflections. In high-aspect-ratio trenches, this leads to , an undercutting effect at the base near the underlying conductive layer due to charge buildup on sidewalls. Pulsed plasma operation mitigates these issues by alternating between active and phases, reducing charge accumulation and improving uniformity, thereby minimizing depths by up to 50% in features with aspect ratios exceeding 10:1. Surface damage and roughness are assessed using techniques like , which measures changes in polarization of reflected light to quantify film thickness loss and infer surface morphology post-etching. spectroscopic , operating in the ultraviolet-visible range, provides real-time monitoring of etch rates and selectivity, detecting roughness increases as small as 1 nm by analyzing psi and delta parameters. This method ensures damage prevention by correlating surface alterations with plasma exposure, guiding process optimization for minimal subsurface implantation damage.

Types

Isotropic Plasma Etching

Isotropic plasma etching is a non-directional dry etching process in which material removal occurs uniformly in all lateral and vertical directions due to the dominance of neutral reactive species, primarily radicals, over ion bombardment. This isotropy arises from chemical reactions at the surface, where reactive radicals, such as fluorine atoms generated from fluorocarbon gases, diffuse to the substrate and form volatile byproducts without significant directional bias from plasma ions. The process is typically diffusion-limited, meaning the etch rate is controlled by the transport of radicals to the surface and the removal of reaction products, leading to uniform etching profiles under high-pressure conditions (e.g., 400 mTorr) that promote multiple scattering of species. In patterned features, this results in undercutting beneath the mask, where lateral etching equals or approaches the vertical etch depth, producing rounded or bowed profiles that are characteristic of radical-driven mechanisms. Common setups for isotropic plasma etching include barrel etchers and downstream plasma systems, which minimize exposure to the to enhance chemical . Barrel etchers position wafers away from electrodes in a high-pressure chamber, allowing random trajectories of neutral species for purely chemical etching, with high selectivity for over . Downstream configurations generate plasma remotely and direct only long-lived radicals to the substrate via a separate chamber, eliminating energetic ions and enabling precise control over etch chemistry; for example, NF₃/Ar plasmas produce radicals that etch at rates up to 2.1 μm/min vertically and 19.2 μm/min laterally, achieving selectivity ratios such as 50:1 for over . Gas mixtures like CHF₃/O₂ are frequently employed for etching , such as photoresists, where CHF₃ provides carbon and for initial deposition control while O₂ addition increases the F/C ratio to boost etch rates and reduce residues, yielding rates around 494 Å/min for related layers but adaptable for organic materials. This etching mode finds primary applications in blanket material removal and surface preparation, such as wafer cleaning to eliminate organic contaminants or thin films without damaging underlying structures, leveraging its high selectivity and low . It is particularly suited for processes requiring uniform thinning or release of microstructures, like in fabrication where undercutting aids in freeing suspended elements, but it avoids deep trench formation due to the lack of vertical directionality. Key limitations include negligible dependence on aspect ratio, as the diffusion of radicals allows consistent etching regardless of feature depth, unlike ion-directed processes; this simplifies control for shallow features but precludes high-aspect-ratio patterning. Etch rate uniformity across the wafer is generally excellent in barrel and downstream systems, often within 4-5% variation, though it can be influenced by gas flow distribution and chamber geometry, requiring careful parameter tuning for large wafers.

Anisotropic Plasma Etching

Anisotropic plasma etching achieves directional material removal primarily through the acceleration of s toward the substrate surface, enabling the fabrication of high-aspect-ratio features with vertical sidewalls in processes. In (RIE), a self-bias voltage develops on the substrate due to the in systems, where the smaller area leads to a higher potential drop, typically on the order of hundreds of volts. This self-bias creates a strong in the plasma sheath adjacent to the substrate, with a magnitude of approximately 10410^4 V/cm, which accelerates positively charged s perpendicularly toward the surface with energies ranging from 20 to 1000 eV, depending on the process conditions. The directional bombardment enhances chemical reactions at the surface, promoting anisotropic by breaking bonds and facilitating volatile product formation, in contrast to the uniform removal observed in isotropic . The flux to the substrate, Γ=nivB\Gamma = n_i v_B, where nin_i is the ion density at the sheath edge and vB=kTe/miv_B = \sqrt{kT_e / m_i}
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